The invention generally relates to a voltage regulator, such as a switching voltage regulator, that adjusts a timing in response to a load transient.
A DC-to-DC voltage regulator typically is used to convert a DC input voltage to either a higher or a lower DC output voltage. One type of voltage regulator is a switching regulator that is often chosen due to its small size and efficiency. The switching regulator typically includes one or more switches that are rapidly opened and closed to transfer energy between an inductor (a stand-alone inductor or a transformer, as examples) and an input voltage source in a manner that regulates the output voltage.
As an example, referring to FIG. 1, one type of switching regulator is a Buck switching regulator 10 that receives an input DC voltage (called V.sub.IN) and converts the V.sub.IN voltage to a lower regulated output voltage (called V.sub.OUT) that appears at an output terminal 11. To accomplish this, the regulator 10 may include a switch 20 (a metal-oxide-semiconductor field-effect-transistor (MOSFET), for example) that is operated (via a voltage called V.sub.SW) in a manner to regulate the VOUT voltage, as described below.
Referring also FIGS. 2 and 3, in particular, the switch 20 opens and closes to control energization/de-energization cycles 19 (each having a constant duration called T.sub.S) of an inductor 14. In each cycle 19, the regulator 10 asserts, (drives high, for example) the V.sub.SW voltage during an on interval (called T.sub.ON) to close the switch 20 and transfer energy from an input voltage source 9 to the inductor 14. During the T.sub.ON interval, a current (called I.sub.L) of the inductor 14 has a positive slope. During an off interval (called T.sub.OFF) of the cycle 19, the regulator 10 deasserts (drives low, for example) the V.sub.SW voltage to open the switch 20 and isolate the input voltage source 9 from the inductor 14. At this point, the level of the I.sub.L current is not abruptly halted, but rather, a diode 18 begins conducting to transfer energy from the inductor 14 to a bulk capacitor 16 and a load (not shown) that are coupled to the output terminal 11. During the T.sub.OFF interval, the I.sub.L current has a negative slope, and the regulator 10 may close a switch 21 to shunt the diode 18 to reduce the amount of power that is otherwise dissipated by the diode 18. The bulk capacitor 16 serves as a stored energy source that is depleted by the load, and additional energy is transferred from the inductor 14 to the bulk capacitor 16 during each T.sub.ON interval.
For the Buck switching regulator, the ratio of the T.sub.ON interval to the T.sub.OFF interval, called a duty cycle, generally governs the ratio of the V.sub.OUT to the V.sub.IN voltages. Thus, to increase the V.sub.OUT voltage, the duty cycle may be increased, and to decrease the V.sub.OUT voltage, the duty cycle may be decreased.
As an example, the regulator 10 may include a controller 15 (see FIG. 1) that regulates the V.sub.OUT voltage by using a fixed frequency, pulse width modulation (PWM) technique to control the duty cycle. In this manner, the controller 15 may include an error amplifier 23 that amplifies the difference between a reference voltage (called V.sub.REF) and a voltage (called V.sub.P (see FIG. 1)) that is proportional to the V.sub.OUT voltage. Referring also to FIG. 5, the controller 15 may include a comparator 26 that compares the resultant amplified voltage (called V.sub.C) with a sawtooth voltage (called V.sub.SAW) and provides the V.sub.SW signal that indicates the result of the comparison. The V.sub.SAW voltage is provided by a sawtooth oscillator 25 and may have a constant frequency (i.e., 1/T.sub.S).
Due to the above-described arrangement, when the V.sub.OUT voltage increases, the V.sub.C voltage decreases and causes the duty cycle to decrease to counteract the increase in V.sub.OUT. Conversely, when the V.sub.OUT voltage decreases, the V.sub.C voltage increases and causes the duty cycle to increase to counteract the decrease in V.sub.OUT. The switching frequency (i.e., 1/T.sub.S) typically controls the magnitude of an AC ripple component (called V.sub.RIPPLE (see FIG. 4)) of the V.sub.OUT voltage, as a higher switching frequency typically reduces the magnitude of the V.sub.RIPPLE voltage.
The regulator 10 may be part of a computer system and thus, may be used to provide power to components, such as a microprocessor, of the computer system. Because of the ever-increasing operating frequency and power requirements of the microprocessor, the microprocessor may consume a significant amount of power. When the power that is demanded by the microprocessor suddenly increases, giving rise to a transient condition, the voltage that is supplied by the regulator 10 may tend to decrease below an acceptable range of voltages. To prevent this from occurring, the computer system may include a significant amount of decoupling capacitors (not shown) to prevent the voltage that supplies the microprocessor from substantially decreasing when the output load of the regulator 10 suddenly changes. Without the decoupling capacitors, the voltage supplied to the microprocessor may drop below an acceptable level due to the above-described PWM control. In this manner, when a significant load transient occurs, the control scheme may be within a dead time interval, a time interval in which the switch 20 is open, thereby preventing energy from being transferred from the input source 9 to counteract the transient. In general, the response of the regulator 10 to a load transient is a function of the inductance of the inductor 14. Although the current in the inductor 14 cannot change instantaneously when the switch 20 closes, in general, the smaller the inductance of the inductor 14, the faster the regulator 10 may respond to counteract the transient. However, if the control scheme is within the dead interval, an additional time elapses in which the regulator 10 cannot respond to the transient.
For example, referring to FIGS. 6 and 7, the controller 15 (see FIG. 1) may generate pulse width modulated pulses 30 (via the V.sub.SW signal) to regulate the V.sub.OUT voltage for a given level (called I.sub.CC--MIN) of output current (called I.sub.OUT) of the regulator 10. At time T.sub.3, the pulse 30a may end, thereby causing the switch 20 to open at time T.sub.3. However, also at time T.sub.3, the I.sub.OUT current may transition from the I.sub.CC--MIN level to a higher current level (called I.sub.CC--MAX). The controller 15 may not close the switch 20 until another switching cycle begins (and until another pulse 30b is generated) at time T.sub.4. Therefore, a dead time interval 32 may occur in which the switch 20 is open, a state of the regulator 10 that prevents the regulator 10 from immediately responding to the increased load.
The duration of the dead interval 32 may be reduced by coupling two of the regulators 10 in parallel and operating their switches 20 in a complementary fashion. However, this arrangement may also not respond fast enough to prevent a significant drop in the regulator's output voltage.
Thus, there is a continuing need for a switching regulator having an improved response to load transients.